282 PEOTECTION AGAINST AVALANCHES. 



erties, it converts tlie supporting layer into a semi-fluid mud, whicli 

 opposes no obstacle to the sliding of tlie strata above. 



The upper part of the mountain which buried Goldau wag 

 composed of a hard but brittle conglomerate, called nagelflue, 

 resting on an unctuous clay, and incKning rapidly towards the 

 village. Much earth remained upon the rock, in irregular 

 masses, but the woods had been felled, and the water had free 

 access to the surface, and to the crevices which sun and frost had 

 already produced in the rock, and, of course, to the shmy stratum 

 beneath. The whole summer of 1806 had been very wet, and an 

 almost incessant deluge of rain had fallen the day preceding the 

 catastrophe, as well as on that of its occurrence. All conditions, 

 then, were favorable to the sliding of the rock, and, in obedience 

 to the laws of gravitation, it precipitated itself into the valley as 

 soon as its adhesion to the earth beneath it was destroyed by the 

 conversion of the latter into a viscous paste. The mass that fell 

 measured between two and a half and three miles in length by 

 one thousand feet in width, and its average thickness is thought 

 to have been about a hundred feet. The highest portion of the 

 mountain was more than three thousand feet above the village, 

 and the momentum acquired by the rocks and earth in their 

 descent carried huge blocks of stone far up the opposite slope of 

 the Rigi. 



The Piz, which fell into the Cordevole, rested on a steeply 

 inclined stratum of hmestone, with a thin layer of calcareous 

 marl intervening, which, by long closure to frost and the infil- 

 tration of water, had lost its original consistence, and become a 

 loose and slippery mass instead of a cohesive and tenacious bed. 



ProtecUon agamist Avalanches. 



In Switzerland and other snowy and mountainous countries, 

 forests render a most important service by preventing the for- 

 mation and fall of destructive avalanches, and in many parts of 

 the Alps exposed to this catastrophe, the woods are protected, 

 though too often ineffectually, by law. No forest, indeed, could 

 arrest a large avalanche once in full motion, but the mechanical re- 

 sistance afforded by the trees prevents their formation, both by 

 obstructing the wind, which gives to the dry snow of the Staub- 



